The invention relates to the field of optics and optical processing. More particularly, the invention relates to a method of shifting the frequency of optical beam, e.g., a beam carrying a telecommunication signal.
Currently, many telecommunication networks send and receive information as optical signals over optical fiber networks. Such optical fiber networks generally provide significantly greater bandwidth than their electrical wire counterparts. One reason for this is that many optical networks presently use wavelength division multiplexing (WDM) techniques in which a single optical fiber can carry multiple (e.g., 80 or more) signals on different wavelength channels simultaneously. Currently, the wavelength channels are generally in the neighborhood of 1.55 microns, and different wavelength channels are separated by an amount on the order of 100 GHz (0.1 THz).
Even with such bandwidth, there can be bottlenecks at various nodes in the optical fiber network where incoming optical signals are rerouted among different pathways. In particular, when it is desired to route an incoming signal to a desired output pathway, the wavelength channel of the incoming optical signal may already be occupied in the desired output pathway. In such cases, the wavelength of the incoming signal needs to be shifted to an available channel or else the connection may be refused. One method for such wavelength shifting is to convert the incoming signal into an electrical signal and then convert the electrical signal into another optical signal using a source at the available wavelength channel. Such electrical conversion is cumbersome, however, and many researchers are looking to all-optical solutions where optical signals are switched directly among selected wavelengths to optimize bandwidth utilization in the network.
Similarly, outside of the telecommunication field, it is often desirable to shift the wavelength of an optical beam to facilitate downstream applications and/or processing. For example, such wavelength shifting is useful in spectroscopy, metrology, photomedicine, and laser-based materials processing.
The invention features a method for shifting the wavelength of an input electromagnetic beam. The method involves generating a polariton wave at a selected frequency in a material supporting such waves, and interacting the input beam with the polariton wave to shift the wavelength (or equivalently, the frequency) by an amount corresponding to the polariton wave frequency. Polariton waves are dispersive and span a wide range of frequencies. Thus, they can be generated to have a selected frequency. Moreover, in many embodiments, the frequency of the polariton wave can be dynamically adjusted to produce a corresponding, tunable wavelength-shift in the input beam. For example, an external electric field may be applied the material supporting the polariton to dynamically adjust the polariton wave frequency. Furthermore, in some embodiments, the polariton wave is generated in a cavity in that resonantly supports the selected polariton wave frequency, and thereby enhances the efficiency, selectivity, and simplicity of the desired frequency shift.
The input electromagnetic beam may be a signal beam carrying information such as a telecommunication signal. For example, the beam may have a modulated intensity profile, such as sequence of pulses representing a digital bit stream. Alternatively, or in addition, the beam may have a modulated phase profile to carry the information. Furthermore, in many such embodiments, the coherent bandwidth of the signal beam is less than the wavelength shift.
In general, in one aspect, the invention features a method for shifting the frequency of an electromagnetic beam. The method includes: generating in a material a polariton wave having a polariton wave frequency; and directing the beam to interact with the polariton wave and cause at least portion of the beam to shift in frequency by an amount corresponding to the polariton wave frequency.
Embodiments of the method may include any of the following features.
The method may further include selecting the polariton wave frequency based on a desired frequency shift for the electromagnetic beam.
The method may further include adjusting the polariton wave frequency to cause a corresponding adjustment to the frequency shift. For example, the adjustment of the polariton wave frequency may include applying an external electric field to the material.
The method may further include adjusting the frequency shift by repeating the generating and directing steps for a polariton wave having a different polariton wave frequency corresponding to the adjusted frequency shift.
The method may further include allowing the polariton wave to propagate in the material prior to its interaction with the electromagnetic beam.
The method may further include introducing a delay between the generation of the polariton wave and its interaction with the electromagnetic beam.
The generation of the polariton wave may be selected to occur at a first spatial region of the material and the interaction between the polariton wave and the electromagnetic beam may be selected to occur at a different, second spatial region of the material.
The material may define a polariton wave cavity resonant with the polariton wave frequency. For example, the material may includes at least one constituent material having a transverse dimension sufficient to define the resonant cavity. Because of the cavity, the method may further include resonantly pumping the cavity to increase the intensity or frequency selectivity of the polariton wave. Furthermore, the method may further include selecting the polariton wave frequency by applying an external electric field to the material to adjust the effective cavity length of the resonant cavity. Also, the material may define an array of cavities each having a resonant polariton wave frequency. In such embodiments, the method may further include selecting a desired polariton wave frequency, and generating the polariton wave in the cavity corresponding to the desired polariton wave frequency.
The frequency-shift may equal the polariton wave frequency or a harmonic of the polariton wave frequency. Furthermore, the frequency-shift may be selected to be positive or negative.
The electromagnetic beam whose frequency is being shifted may carry a telecommunication signal. For example, the electromagnetic beam may carry the telecommunication signal as a modulated intensity profile or a modulated phase profile. Furthermore, the electromagnetic beam carrying the telecommunication signal may have a coherent bandwidth that is less than the frequency shift. Also, the electromagnetic beam may include pulsed electromagnetic radiation or it may include continuous-wave (cw) or quasi-cw electromagnetic radiation.
The electromagnetic beam may be directed to the material by an optical waveguide.
The polariton wave frequency may be in the range of about 50 GHz to about 10 THz.
The electromagnetic beam may have a central wavelength in the range of about 300 nm to 2.5 microns.
The material may include a crystalline material, such as, for example, a ferroelectric or a semiconductor crystal. Furthermore, the material may non-centrosymmetric material.
The generation of the polariton wave may include optically exciting the material. For example, the optical excitation of the polariton wave may include directing at least one pulse of optical radiation to the material, wherein the optical pulse has a pulse duration shorter than the inverse of the polariton wave frequency, e.g., a pulse duration shorter than 10 ps. Alternatively, for example, the optical excitation of the polariton wave may include simultaneously directing at least two optical excitation beams to overlap in the material, wherein any two of the optical excitation beams differ in frequency by an amount equal to the polariton wave frequency.
The optical excitation of the polariton wave may include forming an optical excitation grating pattern in the material. For example, the formation of the optical excitation grating pattern may crossing a pair of optical excitation beams on the material. Alternatively, for example, the formation of the optical excitation grating pattern may include directing optical radiation to a mask and imaging at least a portion of the masked light into the material. In any case, the method may further include adjusting the period of the optical excitation grating pattern to select the polariton wave frequency.
The optical excitation of the polariton wave may also include coupling electromagnetic radiation at the polariton wave frequency into the material.
Alternatively, the excitation of the polariton wave may include directing at least one electrical pulse to the material, wherein the electrical pulse has a pulse duration shorter than the inverse of the polariton wave frequency.
To interact with the polariton wave, the beam whose frequency is to be shifted may be directed into the material supporting the polariton wave. Alternatively, it may be directed to a region proximate the material supporting the polariton wave.
In general, in another aspect, the invention features a method for shifting the frequency of an electromagnetic beam. The method includes: selecting a polariton wave frequency based on a desired frequency shift for the electromagnetic beam; generating in a material a polariton wave having the selected polariton wave frequency; and directing the beam to interact with the polariton wave and cause at least portion of the beam to shift in frequency by an amount corresponding to the polariton wave frequency.
In general, in another aspect, the invention features a method for shifting the frequency of an electromagnetic beam. The method includes: generating in a material a polariton wave having a polariton wave frequency; directing the beam to interact with the polariton wave and cause at least a portion of the beam to shift in frequency by an amount corresponding to the polariton wave frequency; and adjusting the polariton wave frequency to cause a corresponding adjustment to the frequency shift. For example, the adjustment of the polariton wave frequency may include applying an external electric field to the material. Moreover, the material may define a polariton wave cavity resonant with the polariton wave frequency.
In general, in another aspect, the invention features a method for shifting the frequency of an electromagnetic beam. The method includes: providing a material defining an array of polariton cavities each having a resonant polariton wave frequency; generating a polariton wave in the cavity corresponding to a selected polariton wave frequency; and directing the beam to interact with the polariton wave and cause at least a portion of the beam to shift in frequency by an amount corresponding to the polariton wave frequency.
Embodiments of the invention may have any of the following advantages.
The method may be used to facilitate all-optical, wavelength-shifting of photonic optical signals. In other words, the wavelength of an optical signal may be shifted without converting the optical signal to an electrical signal. Thus, such signals may be routed along a network pathway by switching the signal to an available wavelength channel at each of one or more legs of the pathway, thereby optimizing the use of the available network bandwidth. Moreover, the wavelength shift is tunable by generating the polariton wave to have a polariton wave frequency corresponding to the desired frequency shift. Furthermore, a cavity may used to resonantly enhance the polariton wave intensity and control the conversion efficiency of the desired wavelength shift. Also, in those embodiments in which the polariton is generated by optical excitation, that optical excitation may be spatially and/or temporally separated from the interaction between the polariton wave and the EM input beam whose wavelength is to be shifted. Thus, nonlinear interactions between the optical excitation used to generate the polariton wave and the input beam are minimized, if not prevented. Such nonlinear interactions may otherwise produce nonlinear effects (e.g., photorefractive damage) that corrupt that input beam and/or degrade the material supporting the polariton wave.
Other features, aspects, and advantages of the invention will be apparent from the following detailed description, and from the claims.